Lithium ion batteries are being used for mobile grid deployments in cases of natural events (emergencies) and there could be cases where driving long distances in a near zero volt condition would significantly reduce the risk of serious event during an accident.
In addition to safe transport and storage of lithium ion batteries, several other potential benefits exist for cells that can be discharged to and stored at a near zero volt state of charge. For one, a cell would be, by extension, more tolerant to over-discharge during operation that can occur due to mismanagement of a battery system. Individual cells or batteries could also be stored for long periods without regular checking of the open circuit voltage or having to apply trickle charging. This is especially useful for batteries that are difficult to access such as those in implanted medical devices or those used in satellites.
In the case of emergency, it would be useful to have lithium ion cells that could be completely discharged to a near zero volt state of charge with only a resistor and no sophisticated discharging equipment. The cells could then be recovered and used again at a later time. This could be very useful in home energy storage (like a Tesla Powerwall), where a fire alarm could trigger a fixed load to be applied to all cells in a battery and discharge them to a near zero volt state of charge. As long as all cells are completely discharged before a house fire reaches the battery, it is much less likely to go into thermal runaway upon overheating. This would reduce further damage to a home and make the fire much more manageable for emergency crews. The same is true for an electric vehicle in an accident. An accident could trigger an appropriately sized resistor bank/radiator to discharge the cells in the battery to a near zero volt state of charge. Such a battery would reduce risk to vehicle passengers and make handling of the wreckage by emergency crews much safer. In both cases, the battery could then be recovered and re-used if un-damaged by the event or there was a false alarm. This would prevent a substantial loss of assets (given that the cost of the battery is currently 400-600 $/kWh) and reduce the environmental impact of lithium ion battery waste.
Shipping and transportation entities could benefit from a scenario where the safety risks are better controlled prior to shipment. A cell is desired that is stored with a resistor between its leads will be at or near zero volts, and its state of charge can be easily checked by a hand-held volt meter to ensure it is fully discharged. This enables more layers of checks to ensure that lithium ion cells are at a safe state of charge before they are transported, stored, etc. In comparison, voltage checks of batteries may not accurately reflect the true state of charge (SOC) which carries different risk since the actual SOC directly correlates with the exothermic energy that can be released upon abuse. Specifically, the Open Circuit Voltage (OCV) of a cell can vary for a given state of charge during storage, making it an unreliable indicator of the state of charge of a cell; as well as be sensitive to the length of time in open circuit and the environmental conditions such as temperature. The challenge of correlating the OCV with SOC is even more difficult in cells using positive electrode materials that have a nearly constant discharge voltage, such as LiFePO4. To be sure of the state of charge, sophisticated charging equipment and a large amount of time are required, which renders such an approach not viable for shipping organizations like UPS, USPS, and FEDEX.
As society transitions to renewable energy sources and expanded use of electrical energy, energy storage in a reliable and safe manner is becoming ever more paramount. For many portable applications electrochemical energy storage using lithium ion batteries is currently the premier method due to the enhanced rechargeable chemistry. Compared to other designs (i.e., NiCd, Ni—H, etc.) lithium ion has higher energy density, cycle life and highly tunable performance characteristics. There are many efforts underway to enhance lithium ion battery performance to align with future application needs, however, as energy density increases, the safety risks also increase.
When in a user-active state, manufacturer defect or abuse of lithium ion cells within a battery can lead to a thermal runaway event. Thermal runaway results from several internal exothermic reactions that are initiated by overheating of a cell by internal short, rapid discharge, external heating or other abuse condition. The exothermic reactions include SEI decomposition, electrolyte reaction with the electrodes, decomposition of active materials and electrolyte decomposition. Thermal runaway can result in a dangerous fire or explosion that can propagate to other nearby cells. In the case of a large battery consisting of many lithium ion cells or many batteries stored together, this can lead to a very dangerous event with severe damage including explosion, fire, and venting of toxic gases. Several research efforts to mitigate safety risks while lithium ion batteries are in a user-active state have been reported. Efforts internal to the lithium ion cells such as shut-down separators to prevent current flow upon overheating, positive electrode coatings to suppress exothermic release, non-flammable electrolytes to avoid electrolyte combustion and redox shuttle additives to prevent overcharge have been investigated. Efforts external to cells such as Positive Temperature Coefficient devices (PTC) to block or reduce current upon overheating, battery management systems (BMS) to avoid abuse of cells, Current Interrupt Device (CID) to block current in the case of over-pressure, current limiting fuses to prevent rapid charge or discharge, blocking diodes to prevent inadvertent charge or discharge, and bypass diodes to prevent overcharge/overdischarge of a “weak” cell in a battery pack have all been investigated or are currently used.
In the case of a user-inactive state, it would be ideal for the individual cells (and therefore the whole battery) of a lithium ion battery to store no charge energy and be at a zero volt state of charge since the battery does not need stored energy to perform a task. This would minimize the amount of energy that could be released in a catastrophic event, reducing safety risks associated with the inactive battery. Discharging cells only to their normal end-of-discharge cutoff voltage (still some charge remaining) has been shown to prevent thermal runaway under physical abuse conditions (i.e., nail penetration) and lead to higher onset temperatures of thermal runaway/exothermic reactions upon overheating. As such, discharging cells to a very low state of charge before storage and shipping could greatly reduce the safety risks associated with them in an inactive state.
However, discharging conventional lithium ion cells to very low states of charge, especially to near zero volts, risks damaging internal components of the cells (most significantly dissolution of the copper current collector of the negative electrode.). Even if cells are not intentionally discharged to very low states of charge, gradual self-discharge can, over time, bring the cell voltages to damaging low levels. As such, cells are currently shipped and stored in a partially charged state to mitigate the chance of self-discharge bringing the cell voltage too low. In this partially charged state, the energy stored can present a safety risk.
If lithium ion cells could be modified in such a way that discharge to low voltages did not damage internal components, they could be discharged to very low states of charge prior to shipping and storage without concern for performance degradation. As discussed above, this could substantially reduce safety concerns with the cells when in an inactive state.
While this is a promising approach from a safety standpoint, in order for it to be implementable it also needs to be highly controllable in order to comply with regulations. A cell stored at open-circuit, even in a low state of charge, will have an open-circuit voltage that can vary depending on ambient conditions, history of the cell and the active materials used in the cell. As such, checking the open circuit voltage of a cell with a volt meter is insufficient for definitively determining the state of charge of the cell. Instead, sophisticated charging equipment and significant amounts of time would be required. As a result, shipping and storage entities would be unable to rapidly assess the state of charge of a cell on their own and would likely have to rely on manufacturers to ensure that cells are in a specified low state of charge prior to shipment/storage. This could lead to errors that result in unintentional safety risk from cells in a high state of charge.
In an alternative scenario, an appropriately sized resistor (i.e., won't discharge the cell at too high of a rate) could be applied to a cell that is ideally already in a low state of charge. Once applied, it would effectively completely discharge the cell, bringing the cell voltage to near zero volts and maintaining it there as long as the resistor stays applied to the cell. This state could easily be checked with a handheld voltmeter because regardless of ambient conditions, cell history, or active materials used the cell voltage will be at near zero volts. This will enable the state of charge of the cells to be easily checked at multiple stages of transit and storage, adding significant controllability to ensuring a safe state of charge of the cell.
Enabling lithium ion cells to an applied resistor at near zero volts without damaging internal components could present a highly controllable way to effectively mitigate the safety risks associated with storing and transporting them, especially when in large battery packs and/or large cell formats. This could lead to greatly reduced restrictions on their storage and transportation, which would reduce costs and increase the distribution network of lithium ion batteries. Additional safety capabilities such as this could also help to assuage public concerns over the safety of lithium ion batteries, especially in emerging home and electric vehicle applications. The key with such a promising approach is to accomplish tolerance to near zero volt storage with little to no modification to a conventional lithium ion cell design; which would be a stark contrast to past approaches that require modifications to cell design and use of unconventional materials that can reduce cell quality and performance.
One strategy to avoid copper dissolution during zero volt storage has been to employ alternative negative electrode current collectors which do not undergo dissolution at higher potentials vs. Li/Li+. Voltammetry studies have been done previously on potential current collector replacement materials to determine the stability of the material at high potentials vs. Li/Li+ (i.e., >3V) as well as low potentials vs. Li/Li+ (i.e., 5-1000 mV), which is a requirement for negative electrode current collectors. Some metals, like aluminum, will alloy with lithium at low potentials in conventional LiPF6-based electrolytes which leads to negative electrode pulverization during normal cycling in this voltage range. Titanium, titanium alloys, nickel, nickel alloys and stainless steel have all been patented as potential candidates because they meet the high and low voltage stability requirements.
Titanium foils are showing promise in commercially developed zero volt storage capable cells as a negative electrode current collector. However, some drawbacks exist with typical titanium foils in that they are typically thicker than standard copper foils and can cost substantially more. This can substantially reduce both the volumetric and gravimetric energy density of the cells while increasing cost. Additionally, bulk titanium is more than an order of magnitude more resistive than copper, which has been stated to limit the rate capability of cells, especially in larger format or wound cells.
Carbon nanotube and graphene free-standing electrodes may also be potential current collector replacements due to their high chemical stability. Negative electrodes made purely of other carbon allotropes have already been demonstrated to generate cells that can tolerate constant-load near-zero-volt and overdischarge conditions. However, lower bulk electrical conductivity, coulombic efficiency issues from S E I (SEI) formation on nanoscale surface, and higher cost are disadvantages of the nanocarbon-based current collectors compared to copper.
In addition to utilizing replacement current collector materials for copper, another strategy to protect cells during overdischarge conditions is to passivate copper current collectors and prevent dissolution at high potentials vs. Li/Li+. One approach is the use of succinonitrile as an electrolyte additive to passivate the copper current collector and prevent its corrosion. However, succinonitrile has also been shown to significantly increase the impedance of the positive electrode during cycling. Formation of nitrile compounds on the surface of copper foil before electrode fabrication has also shown promise, but it is unclear what effect this might have on the charge transfer resistance between the copper current collector and negative electrode composite.
Another past approach to zero volt storage tolerance of lithium ion cells has been to appropriately modify the cell with secondary active materials in either the positive electrode, negative electrode or both that have charge and discharge characteristics to prevent high negative electrode potential vs. Li/Li+ during overdischarge or near zero volt storage. No widely available validation or experimental data exists on many of the patented secondary active materials. Thus, the resilience to multi-day near zero volt storage and impact on performance during normal operation is unclear.
Overall, use of secondary active materials in the electrodes has major potential drawbacks. Namely, any approach that adds significant amounts of secondary active materials with intermediate charge/discharge potentials will likely decrease cell energy density by lowering the average discharge voltage and/or electrode specific capacity. Concerns for differing rate capability effects between primary and secondary materials could limit this approach. Additionally, many operational concerns exist over the stability of secondary active materials in the potential range of the primary active materials as well as the repeated cycling behavior is unknown.